This invention relates to magnetic resonance (MR) data acquisition. More particularly, it relates to methods for limiting the volume of interest and for simultaneously acquiring multiple imaging volumes during MR studies.
An MR imaging system provides an image of a patient or other object in an imaging volume based on detected radio frequency (RF) signals from precessing nuclear magnetic moments. A main magnet produces a static magnetic field, or B0 field, over the imaging volume. Similarly, gradients coils within the MR imaging system are employed to quickly switch into effect magnetic radiance along mutually orthogonal x, y, z coordinates in the static B0 field during selected portions of an MR imaging data acquisition cycle. Meanwhile, an RF coil produces RF magnetic field pulses, referred to as a B1 field, perpendicular to the B0 field, within the imaging volume to excite the nuclei. The nuclei are thereby excited to precess about an axis at a resonant RF frequency. As the precession occurs into the transverse plane, the transverse component of magnetization is magnetically coupled to some external circuitry, typically a receiver. These transmitter and receiver coupling mechanisms both are called RF coils.
RF antennas or coils are tuned and resonate in a frequency band defined by the Larmor frequency and the presence of a gradient field. The filling factor for RF coils is defined as the volume of sensitivity for a given RF coil. In general, the RF coil should be completely filled by the subject, as to eliminate unwanted noise sensitivity from the larger volume. This filling factor is very important and is inversely proportional to signal to noise ratios (SNR). A transmitter coil or body coil, is designed to be uniformly sensitive over an entire field of view (FOV) as defined by the system. This design provides flexibility for imaging large or small volumes.
Most current MR imaging techniques require separate, sequential acquisitions for each volume of interest when multiple volumes are of interest. This invention describes a technique for simultaneous acquisition of separate volumes of interest from different groups of RF coil elements where each group of coil elements has a higher sensitivity to one particular volume of interest than to the other volumes. This technique allows for individualized shim settings to be used for each volume during the spin preparation part of the imaging sequence. For sequences which have a fat suppression technique as the spin preparation, the ability to use individualized shim settings for each volume is crucial to obtaining good image quality.
For clinical studies using contrast agent where more than one volume is of interest following the injection of the contrast material, currently available three dimensional gradient echo techniques require that the volumes be imaged sequentially, or that a large enough volume is prescribed to encompass both regions. The first alternative results in different contrast characteristics for the volumes, and the possible loss of important early contrast uptake information for the volumes that are acquired later. The second alternative compromises the spatial resolution of the study, and is an inefficient way to acquire data if the regions of interest are not close together. Moreover, since it is often necessary to suppress the signal from fat to improve conspicuity of the areas of contrast agent uptake, the use of a larger volume usually results in compromised fat suppression due to the reduced efficacy of main field correction techniques for larger volumes.
One example of a clinical application where simultaneous acquisition of high-resolution images from separate volumes is desirable is MR breast imaging. Currently, approaches to this problem are: either a sequential acquisition of two sagittal volumes corresponding to the left and right breasts separately, or, acquisition of a single axial or coronal volume that encompasses both breasts. The sagittal orientation is preferred for breast imaging because those images correlate better with mammography, and it offers the most efficient filling of imaged volume with breast tissue. Use of an axial or coronal volume to encompass both breasts compromises the spatial resolution of the image, and is inefficient because it requires the acquisition of data corresponding to the empty space between two breasts. In addition, when imaging both breasts simultaneously with one large FOV, the main field homogeneity corrections applied (shim currents) are set to a single value for both volumes, a compromise that reduces the quality of fat suppression. The quality of fat suppression could be improved by using two different sets of shim currents optimized for the left and right breasts respectively during the corresponding spin preparations. This invention provides a method for simultaneous acquisition of two sagittal volumes, and also incorporates a technique for optimizing the quality of fat suppression (or other spin preparation) over both volumes.